MOMENTUM • VIRGINIA TECH MECHANICAL ENGINEERING 21
How a beetle ’ s colored armor works
Li and his team launched their research from knowledge of a beetle ’ s shell composition : their outer exocuticle layer contains a unique microstructure with only 1 / 30 of a millimeter thickness . Its composition is a stack of horizontal nanoscale layers inserted with vertical microscale pillars , providing the exoskeleton with optical coloration and mechanical strength at the same time .
Unlike pigment-based colorations , the optical appearance of the flower beetle results from the exoskeleton ’ s microstructure . The nanolayered region consists of two alternating material compositions , which selectively reflect light of certain colors . This phenomenon is called structural color or photonic color .
Structural color is a common strategy to produce coloration in nature , as seen in butterfly wings , bird feathers , and even some plants and mollusk shells . In 2015 , Li and his colleagues discovered that a type of limpet found in Europe develops its iridescent blue color in its shells through a similar multilayered microstructure out of the mineral calcite , the same material found in chalk .
In addition to providing coloration , the exoskeletal shell of beetles needs to be strong and damage tolerant , Li explained . The flower beetle achieves this through reinforcement of its shell ’ s vertical micropillars . When the microstructure is pierced , the shell ’ s micropillars hold a seal around the site of the puncture . This prohibits the beetle ’ s wing from tearing , cracking , or delaminating . The micropillars are also able to spring backward , thus reducing the size of the damage site intruded by the incoming object after unloading .
The micropillar-reinforced nano-multilayer structure in the exocuticle layer of the flower beetle . The multilayer forms the structural color and the micropillars bring extra mechanical and optical benefits .
Micropillars with multiple jobs
Knowing that the mechanical and optical functions were linked , the team sought to discover which of the two were primary .
By collaborating with Mathias Kolle from MIT , the team developed an optical modeling program to simulate the optical response of the beetle ’ s microstructure . They found that the presence of micropillars , while reducing some degree of optical reflection , is able to redistribute the reflected light to a greater angular range . This contributes to the beetle ’ s